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Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Related Party Transaction [Line Items]
Net premiums written	$ 4,348	$ 3,781	$ 8,863	$ 7,866
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	9,044	$ 7,473	9,044	7,473	$ 8,969	$ 8,260	$ 7,509	$ 7,064
Reinsurance balances payable	2,841		2,841			2,137
Other assets	6,684		6,684			6,231
Other liabilities	3,225		3,225			3,039
Operating affiliates | Premia
Related Party Transaction [Line Items]
Funds held asset	127		127			137
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written			419	$ 428
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,800		1,800			1,600
Reinsurance balances payable	602		602			$ 489
Equity Method Investee | Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Other assets	290		290
Other liabilities	$ 290		$ 290

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Amount of the reinsurer's funds held on its behalf by the ceding insurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of the obligations thereunder.

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